The difference in one sentence
PPI prices what a producer sells at, at the first commercial transaction. CPI prices what a consumer buys at, months later, with distribution and retail markup layered on top.
Why the mechanics differ
PPI tracks the price a producer receives for an input at the point it first enters commerce: a steel mill selling coil, a chemical plant selling resin. That's close to the actual cost driver a B2B purchase contract is trying to track.
CPI tracks the price a consumer pays at final retail purchase, which includes everything added between the producer and the shelf: distribution, retailer margin, and a different, much broader basket weighted toward services and consumer goods rather than industrial inputs. A material's CPI reading, where one even exists, is not the same number as its PPI reading, and the gap between them isn't stable. It moves with retail margin and distribution cost independently of what the producer actually charged.
The table compares CPI-U with two producer series over the same 12 months. The producer series are the PPI for finished goods and the PPI for industrial commodities less fuels.
Over the 12 months to Sep 2026, these series moved differently. On the same $250,000, the gap between the highest and lowest is $11,676.
| Series | What it covers | 12 months | On $250,000 |
|---|---|---|---|
CUUR0000SA0 | All Items, US City Average | +3.7% | +$9,344 |
WPUFD49207 | Finished Goods | +8.4% | +$21,021 |
WPU03T15M05 | Industrial Commodities Less Fuels | +7.6% | +$19,013 |
When PPI is the right choice
Almost any contract indexing a purchased material or component should use PPI, or an industry-specific PPI series where one exists. It's the only one of the two that actually isolates the input's production cost from everything layered on top of it downstream. This is also often a hard constraint rather than a preference: most raw materials and industrial components (steel, aluminum, resin, cement) have no meaningful retail CPI reading at all, because consumers don't buy them directly. PPI is the only series that exists to index against.
When CPI is the right choice
CPI fits cases where the contract genuinely wants to track what an end buyer or worker experiences, not a producer's input cost: labor cost-of-living escalators (see labor escalation clause for the Employment Cost Index alternative), rent and lease escalators, or a consumer-facing service contract where the whole point is matching what customers are actually paying elsewhere.
Take a contract signed in September 2025 with $250,000 of rent or service scope, invoiced in September 2026. The table applies the change in CPI-U (series CUUR0000SA0) to that amount.
| Base index, Sep 2025 | 324.8 |
| Current index, Sep 2026 | 336.94 |
| Ratio, 336.94 ÷ 324.8 | 1.0374 |
| Amount at signing | $250,000 |
| Adjustment added to that amount | +$9,344 |
Live figures from the stored series, updated when the provider publishes.
Which to choose
Default to PPI, or the relevant industry PPI series, for any contract indexing a material or component's production cost. Check the vertical guides on this site for the specific series. Use CPI only when the underlying cost genuinely is a downstream, consumer-facing, or labor cost-of-living driver, not a producer input.